An electromagnetically driven valve employed as an intake valve or an exhaust valve of an internal combustion engine is disclosed, for instance, in Japanese Patent Official Publication No. HEI 4-502048 and Japanese Patent Application Laid-Open No. HEI 7-335437. This electromagnetically driven valve is provided with an armature attached to a valve body. An upper spring and a lower spring are disposed above and below the armature respectively. These springs urge the armature toward its neutral position.
An upper core and a lower core are disposed above and below the armature respectively and an upper coil and a lower coil are disposed within the upper core and the lower core respectively. The upper coil and the lower coil, if supplied with an exciting current, generate a magnetic flux circulating therethrough. Upon generation of such a magnetic flux, the armature is attracted toward the upper core or the lower core by an electromagnetic force (hereinafter referred to as an attracting force). Thus, the aforementioned electromagnetically driven valve can displace the valve body to its closed position or its open position by supplying a predetermined exciting current to the upper coil or the lower coil.
If supply of an exciting current to the upper coil or the lower coil is stopped after displacement of the valve body to its closed position or its open position, the armature and the valve body are urged by the springs to start a simple harmonic motion. Unless the amplitude of the simple harmonic motion is damped, the armature and the valve body that move from one displacement end toward the other displacement end (hereinafter referred to as a desired displacement end) reach the desired displacement end solely due to urging forces of the springs. However, such displacement of the armature and the valve body causes energy loss resulting from sliding friction or the like. Therefore, the critical position that can be reached by the armature and the valve body due to the urging forces of the springs is not coincident with the desired displacement end.
The aforementioned electromagnetically driven valve can compensate for the amount of energy loss resulting from sliding movement and displace the armature and the valve body to the desired displacement end by starting to supply an exciting current to one of the upper coil and the lower coil at a suitable timing after stoppage of supply of an exciting current to the other of the upper coil and the lower coil. The valve body can thereafter be opened and closed by alternately supplying an exciting current to the upper coil and the lower coil at suitable timings.
In the aforementioned electromagnetically driven valve, each of the upper core and the lower core is provided with an annular protrusion disposed along an outer periphery thereof. The annular protrusion, which has a predetermined length, protrudes from an end face of the upper core or the lower core. The inner diameter of the annular protrusion is slightly larger than the outer diameter of the armature.
When the armature is spaced apart from the desired displacement end, the attracting force acting on the armature (hereinafter referred to as a spaced-state attracting force) is larger in the case where the annular protrusion is provided than in the case where the annular protrusion is not provided. On the other hand, when the armature is close to the desired displacement end, the attracting force acting on the armature (hereinafter referred to as a close-state attracting force) is smaller in the case where the annular protrusion is provided than in the case where the annular protrusion is not provided. Accordingly, as the armature approaches the desired displacement end, the aforementioned electromagnetically driven valve can gradually increase an attracting force acting on the armature.
The armature collides with the upper core or the lower core upon arrival of the valve body at its open position or its closed position, thus causing impact noise. In order to reduce impact noise, it is desired to prevent the attracting force acting on the armature from becoming unsuitably large upon arrival of the armature at the desired displacement end.
In order to reliably displace the armature to the desired displacement end, it is necessary to ensure a spaced-state attracting force of a certain magnitude. In order to ensure a large spaced-state attracting force and reduce impact noise in the electromagnetically driven valve, it is advantageous to avoid an abrupt increase in the attracting force acting on the armature as the armature approaches the desired displacement end. The aforementioned electromagnetically driven valve can satisfy the aforementioned advantageous condition during both the valve opening operation and the valve closing operation. As a result, the aforementioned electromagnetically driven valve can achieve an enhanced tranquility.
In the aforementioned electromagnetically driven valve, the neutral position of the armature is set to the central position between an electromagnet on the valve opening side and an electromagnet on the valve closing side. Thus, there is no change in the amount of energy stored in a pair of springs regardless of whether the armature is positioned on the electromagnet on the valve closing side or on the electromagnet on the valve opening side. In this case, there is no substantial change in the amount of energy required for the springs to urge the armature regardless of whether the valve moves in the valve opening direction or in the valve closing direction.
However, the load applied to the valve body in the internal combustion engine may differ depending on whether the valve body moves in the valve opening direction or in the valve closing direction. Hence, a difference in the amount of energy loss may arise depending on whether the valve body of the electromagnetically driven valve moves in the valve opening direction or in the valve closing direction.
For example, the exhaust valve is opened when a high combustion pressure remains in a combustion chamber and it is closed when the combustion pressure is released. In this case, the load applied to the exhaust valve is larger during the valve opening operation than during the valve closing operation.
Preferably, there should be no substantial difference between the exciting current to be supplied to the electromagnet on the valve opening side and the exciting current to be supplied to the electromagnet on the valve closing side.
The aforementioned electromagnetically driven valve is unable to achieve appropriate operating characteristics during the valve opening operation and during the valve closing operation while substantially supplying an equal exciting current to the electromagnets on the valve opening side and on the valve closing side.